The invention relates to a method for NC-controlled grinding of the circumferential edge and top facet of an ophthalmic lens in accordance with the shape of a selected lens mount and the personal optometric data of the lens wearer.
For aesthetic reasons, the top facet on the circumferential edge of a finished ophthalmic lens should have the same distance from the leading edge of the lens all around the circumference of the lens, even if the ophthalmic lens has a shape that differs sharply from the circular.
If the front of the ophthalmic lens is purely spherical, a top facet made with a constant distance from the leading edge of the ophthalmic lens also lies on a spherical surface with the same midpoint as the spherical surface of the front surface of the ophthalmic lens. If an ophthalmic lens is produced by copy grinding according to a pattern disk corresponding to the lens mount, a constant distance of the top facet from the front surface of the ophthalmic lens can be achieved either by resting a feeler on the front surface of the lens mount and controlling the relative displacement of a grinding wheel relative to the ophthalmic lens in the axial direction, or by producing the axial displacement by means of a so-called Panhard rod, whose length corresponds to the radius of the spherical front surface of the ophthalmic lens less the distance of the top facet from the leading edge of the ophthalmic lens.
In NC-controlled grinding of the circumferential edge and top facet of an ophthalmic lens, the shape of the ophthalmic lens and the personal optometric data of the ophthalmic lens wearer are available in the form of a volume of data by which the ophthalmic lens edge grinding machine can be controlled in such fashion that an ophthalmic lens blank is ground in accordance with the shape of the selected lens mount and the top facet is placed on the circumferential edge of the ground ophthalmic lens in such fashion that it extends around the entire circumference at the circumferential edge and/or maintains a certain distance from the leading edge of the ground ophthalmic lens.
In order to acquire the data relating to the shape of the lens mount, it is known that the lens mount can be placed in a device for scanning the facet groove and recording the data in three-dimensional form. It should be noted in this regard that the facet groove in a lens mount, as a result of manufacturing tolerances or deviations in shape, need not necessarily lie on a spherical surface.
A device for three-dimensional scanning of lens mounts is described in German Utility Model G8608291.4 of the Applicant.
In German Offenlegungsschrift 3410040 an NC-controlled ophthalmic lens edge grinding machine is also described. The outer and inner surfaces of an ophthalmic lens blank which is clamped in the ophthalmic lens edge grinding machine are scanned on a curve that matches the shape of the ophthalmic lens. Two spring-mounted rod-shaped feelers constantly rest directly on the glass and their axial position is fed to potentiometers the values and data collected by these potentiometers are transmitted to a computer or a data storage device that uses them to calculate the spatial pattern of the leading and trailing edges of the ophthalmic lens as well as the respective thickness of the glass. From a stored volume of data on different top facets, the computer selects the top facet that can be provided on the circumferential edge of the finished ophthalmic lens without leaving the circumferential edge at any point.
When an ophthalmic lens is measured using this method or a method described in German Patent 3842601 of the Applicant, in which the leading and trailing edges of a pre-ground ophthalmic lens are measured by a scanning head, deviations of the ophthalmic lens from the spherical shape enter into the measurement. Such deviations can result from prismatic or cylindrical grinding superimposed on spherical grinding and also from the location of a near portion. In addition, the decentration of the optical axis of the ophthalmic lens relative to the geometric axis of the ophthalmic lens or the geometric midpoint of the lens mount plays a role.
When an ophthalmic lens ground under NC-control on the basis of information from a device for scanning the facet groove of a lens mount, and the corresponding top facet is then subjected to final grinding, a computer check must be performed to determine whether a path of the top facet, determined in accordance with the measured facet groove, runs along the entire circumferential edge of the finished ophthalmic lens. This especially must take into account the fact that the top facet is intended to be located, everywhere if possible, at a constant predeterminable distance from the leading edge of the ground ophthalmic lens.
It has now been found that an ophthalmic lens ground under these assumptions and provided with a top facet cannot always be simply fitted into the selected lens mount. This requires a finishing of the ophthalmic lens that is time-consuming and requires a certain degree of skill.